Wireless signaling system



Feb. 5, R935. A. H. REEVES 1,989,770

WIRELESS SIGNALING SYSTEM Filed May 16, 1932 3 Sheets-Sheet l TRANSMITTING Low FREQUENCY ANTENNA T7. BFA AMPLIFLIER TRANSMITTER F BAND PASS FILTER POWER AMPLIFIER OSCILLHTOR BALANCED 0561 M01 MODULATOR POWER AMPLIFIER BRND PASS POWER; L. T R HTTENUATOR F FILTER AMPLIFIER 055 ATT A BAND PAss AMPLIFIER FILTER E COUDLJNG- CIRCUIT OSCILLATOR osc c HARMONIC 3 1 MD BALANCED GENERATOR Ham 2 MODULATOR AND MODULATOR BAND PASS cou urve 0 F FILTER CIRCUIT AMPLIFIER A: MA STER M056 11V VE/VZ'OI? OSCILLATOR 4656 I66 REA Es ATTORNEY Feb. 5, 1935.

A. H. REEVES 1,989,770

WIRELESS SIGNALING SYSTEM Filed May 16, 1932 5 Sheets-Sheet 2 BEAT FREQUENCY 1 AR g OSCILLATOR RECEIVING OSC F ANTENNA FIRST DETECTOR DAMPING- I DAMPING- CR CR cIQcuI-I' 5 AMI FIRST INTER- 1 MEADIATE' :Ra-

QUENCY AMPLI- EAL-Fk QQED nrrsc- I plER TORS DAMPING CIRCUIT B056 BDEC 5 BF BAND PASS 1 FILTER FILTEIQ DAMPIMG ATT c cu -r ATTENUATOR R I 4 5ECOND RECTIFIER F AMI IMTERPIEADIATE A FILTER 2 FRE UEN AMPLIFIER BEAT v QSCILLR AMPLIFIER TOR 05c 5ECOND 2 DETECTOR AMPLIFIER 050 AM DETECTOR l AMPLIFIER we TECTOR FILTER mm? F 05% F FILTER osc DEM OSCILLATOR DEMDDULATOR OSCILLATOR F FILTER LOW FREQUENCY AMPLIFIER TR INVENTOR TELEPHONE IzzQEII/EI? Al F162.

4 TTORNEY Feb. 5, 1935. A. H. REEVES 1,989,770

WIRELESS SIGNALING SYSTEM Filed May 16, 1932 3 Sheets-Sheet 3 I, v L W DETEQ OR RECTIFIER 4 J51) R AMPLIFIER RECEIVING- r ANTENNR TRANSFORMER.

DIFFEREN-HAL z HIGN FRE U N Y DETECTOR f HFA AMPL'FIEK BEAT V I QSQILEATOR TRANS E T FILTER FQRM R Hm B 3 A... AL 'vvv TIME OONS7'ANT DEVICE DECTOE ll IL P III I Cz C1 DETECTOR DTC AI V PLIFIER uEoToR LI P FEEQUENC'Y DEC; QQNTRO PCT TUBE AVIPLIFIER 2'22 15; ll BF F'LTER 0 cl L 1 c S L ATOR CT F6 2-O% B MI I' f- BALANCEDES b I a nE onww b DEM EMQDULATOR CONTROL 05cm..- 1: LATOR TIME I f -6. Puma P AMPLIFIER FREQUENCY TCT CONTRQL- TUBE TELEPHONE I RECEIVEK TR NVENTOR.

1445c h. REEVES ATTORNEY Patented Feb. 5, 1935 UNITED STATES PATENT OFFICE WIRELESS SIGNALING SYSTEM New York, N. Y.

Application May 16, 1932, Serial No. 611,546 In France May 20, 1931 23 Claims.

The present invention relates to improvements in signaling systems using modulated waves and more particularly to systems of this type in which the carrier wave is partly or entirely suppressed at the transmitting station.

In systems of this type heretofore suggested, use has been made of various means to keep the frequency of the locally generated carrier wave at the receiving station in synchronism with the frequency of the suppressed or partly suppressed carrier wave at the transmitting station.

The degree of precision with which the oscillators, at the transmitting and receiving stations, have to be synchronized very rapidly increases when the wave length decreases. For instance, in the case of a wavelength of the order of several thousand meters, the necessary precision is of the order of 1/ 3,000, while in the case of waves of the order of several decameters the necessary precision is of the order of 1/ 1,000,000.

To synchronize the oscillators employedat the sending and receiving stations, it has already been suggested to make one of the oscillators dependent upon the other by using a pilot frequency transmitted from the sending station for controlling the frequency of the oscillator supplying at the receiving station the carrier frequency used for demodulation purposes. It has also been suggested to compensate for slow variations in the transmission strength of the pilot frequency by providing an automatic regulating amplifier adapted to maintain the pilot frequency at a constant strength. Such systems however have important drawbacks, particularly when used on radio links subjected to fading, since in this case usually the fading phenomena differently affect the frequencies of the signals transmitted and the pilot frequency and distortions result. Further, if, under the influence of fading, the pilot frequency falls below the noise level, the controlled oscillator at the receiving station may undergo undesirable variations in frequency, thus resulting in distortions of the demodulated signals.

It should be noted that in certain signaling systems such as facsimile and television transmission systems on a single side band, it is necessary, not only to keep in synchronism the frequency of the carrier wave at the sending station the frequency of the local carrier wave generated at the receiving station, but also to keep a constant phase relation between the oscillators at the sending and receiving stations in order to avoid distortions of the wave form of the signals transmitted. It is clear that systems as heretofore proposed do not provide a constant phase relation between the synchronized oscillators.

One of the objects of the present invention resides in overcoming the above mentioned drawbacks; another object consists in providing means for minimizing the influence of fading and for insuring a high degree of quality in a signaling system of the type set forth.

In order to obtain a sufliciently accurate control of the local oscillator at the receiver, in spite of fading of the pilot wave, it is necessary to arrange so that during the periods when the said oscillator is no longer controlled (on account of fading of the pilot wave), the difference of frequencies between the two oscillators must not exceed a limit beyond which the distortion of the transmitted signals would be intolerable; for this purpose, delay net-works or devices may be provided in the path of the pilot wave, in order to prevent the harmful effects of a momentary disappearance of the pilot wave. According to a feature of the present invention, the delay devices provided may be either of constant delay or with variable delay in an intermittent manner, or even of variable delay in a continuous manner, and in the latter case they may be controlled in accordance with one or more of the operative conditions of the signaling system, that is to say in accordance with the fading phenomena to which the system is submitted; the delay devices may be controlled in accordance with the amplitude of the received pilot wave, so as to be very short when the received pilot wave is at a fairly high level, but to be very long when the received pilot wave is at a very low level or when it fades out. The provision of such delay devices in a signaling system of the type set forth is a basic feature, in view of the importance of fading phenomena, particularly in the case of short wave transmission.

The above mentioned pilot wave may consist of one or more frequencies or hands of frequencies (that is to say, frequencies regularly spaced in a band of the frequency spectrum) transmitted to the receiving station, where they are employed to control the operative conditions of the receiver. The pilot signal may be transmitted either in a continuous manner or periodically or at different intervals, during the moments when the signals are not transmitted. Moreover, the position of the pilot signal, particularly when this pilot consists of a plurality of frequencies, should be taken into consideration as well as its relative position with respect to the signal frequency band employed in order to prevent harmful inter-modulation. The pilot signal employed between the (1) The maximum energy in the voice ee quency range is in the low frequency zone below 500 cycles; it is consequently desirable. to remove the pilot signal as far as possiblefrom thesehigh power frequencies. In order to avoid increasing the width of the frequency band,.it.has been found convenient to maintain a spacing of 490 to 500 cycles between the transmitted voice frequency band and the pilot signal.

(2 Such signals (voice frequency band and pilotsignal) after modulation are displaced in thefrequencyrange by 400 cycles and the result is that when the signals are received on an ordinary receiver, the result is unintelligible, which insures certain secrecy in the communications. H I

The pilot wave may be employed in different ways to control the local oscillator provided at the-receiver transmitter, for example:

(l)-"I'he pilot wave, after demodulation and amplification; is-employed as a local carrier'wave, either directlywhen a portion ofthe carrier wave of thetransmitting station is transmitted, or indirectly by addition or subtraction, with other suitable frequencies.

I (2) Thefpilot wave' niaybe employed to synchronize a local oscillator by acting on one or more of the elements determining the frequency of this local oscillator. i

"In order to obtain a constantphase relation between the synchronized oscillators, advantage maybe taken of a 'propertyof coupler oscillators consisting in this that, when a synchronizing frequency interacts with an oscillator of approximately the same frequency, the oscillator will, under certain conditions, tend'to adjust itself to oscillate at the same frequencyas the applied synchronizing frequency. It may be noted that the phase relation of the applied frequency and that of theoscillator should bear a certain definite relation so that the oscillations are stable. When it is desired to have a phase relation very finely adjusted, this may be done by providing in the output circuit of the oscillator any suitable type of phase correcting circuit such as, for instance, a tuned circuit which may be slightly detuned, the degree of detuning of this circuitbeing controlledso as to obtain the required fine phase adjustment.

As the amount of control provided by the'synchronizing frequency varies substantially linearly'with the amplitude of the synchronizing frequency, it is accordingly necessaryto provide a gain control equipment adapted to keep the amplitude of the synchronizing frequency constant. Since in the case of wireless transmission the pilot wave may fade out entirely during a very large number of .cycles of the controlled oscillator, the latter may drift in frequency to such an extent that the constant "phase relation may be "lost, this resulting in intolerable distortion. In this case, advantage maybe taken of the fact that fading does not in general equally affact the various frequencies transmitted and accord-ingly a plurality of pilot frequencies may be used so that if one of these pilot frequencies fades out, the remaining pilot or pilots will maintain constant the frequency and phase relation of the controlled oscillator. Since the pilots are necessarily of different frequencies, it is necessary to provide at the receiving station means adapted to bring the frequency and phaserelation of the pilots at a constant value corresponding to that of the oscillator used at the transmitting station.

- This may be done by means of separate oscillators, for instance, in the case of two pilots, the

frequency of the second pilot is brought back to that of the firstby means of a separate oscill'ator controlled exactly in frequency and phase by the beat note between the two pilots.

It should be noted that when one of the pilots fades out the beat note vanishes and it is therefore necessaryto provide a time constant of sufficient value so that the oscillator controlled by the note does not alter its frequency by more than a predetermined amount, that is to say, a small fraction of'one cycle. This is possible in the case of a low frequency oscillator because one cycle of its frequency corresponds to a large num ber of cycles of the carrier wave, whereas it would be impossible-if this method were applied directly on the resuppliedcarrierfrequency. The time constant of the low frequency oscillator is chosen so that, in the absence of the beat note, the oscillator will drift in frequency at the rate of say one tenth of a cycle per second or less. Another reason for using a'low frequency oscillator controlled by the beat note is that the frequency and phase relation of the beat'note vary much more slowly than those of the pilot frequencies.

As the amount of control depends uponthe amplitude of the controlling frequency, and as the amplitude of each one of the pilots may be affected by fading, it may be necessary to provide a gain control equipment for controlling separately and independentlythe amplitude of the controlling beat note.

Broadly, according to the present invention, the effect, in a system of the type set forth, of the pilot frequency or frequencies. on'the controlled oscillatohis adapted-to depend upon the amount of attenuation imposed on the signal by fading phenomena to which the system is subjected for instance means (e. g. delay networks with variable delay, retarded gain control equipment controlling the amplitude of the pilot, since the amount of control depends upon the amplitude of .the pilot) 'may be provided and adapted to delay suitably and according to fading conditions the action of the pilot frequency or fre- .quencieswon the controlled oscillator, so that a constant frequency and/or phase relation are (or is) maintained between the synchronized oscillators. Since the average volume of the signal transmitted may not be aifectedby fading conditions attenuating'the pilot wave to an extent of say 40 to 50 decibels, it is often necessary to provide a quick acting gain control equipment in the path of the pilot wave and a comparatively slow acting gain control equipment in the path of the signals.

In order to show clearly the nature and various, aspects of the invention, a detailed description of a transmission system employing features of the invention will be given hereinafter with reference to the accompanying drawings in which:' 1 d I Fig. 1 shows a singleside band transmission system for short waves; 7

Fig. 2 shows the corresponding receiving station in which means are provided to maintain a constant frequency relation between the oscillators at the transmitter and at the receiver;

Fig. 3 represents a receiving system in which means are provided for maintaining a constant frequency and phase relation between the oscillators at the transmitting and receiving stations.

The transmitter shown in Fig. l is a transmitter rendering it possible to transmit in a radio signaling system a single sideband approximately without distortion. The signals, for example, a telephone conversation, are applied to a transmitter TT whose current is amplified by means of a low frequency amplifier BFA. The amplified currents leaving BFA pass through a bandpass filter F1 permitting the transmission of a frequency band comprised, for example, between 0 to 3 kilocycles. The frequency band thus obtained is applied to a balanced modulator MDi, which is associated with a suitable oscillator OSCi, whose frequency is chosen in such a manner that among the products of modulation of the balanced modulator MDl there is a modulation band comprised, for example, between 16 and 19 kilocycles, corresponding to the signals to be transmitted. This band is filtered by means of a second filter F2.

In parallel on the output of the filter F2 is an oscillator OSCz followed by an attenuator ATT which enables currents of the frequency of the oscillator OSCz but of suitable amplitude to be applied in the output circuit of the filter F2. The currents produced by this oscillator serve as a pilot wave.

The oscillator OSCz may have a frequency outside the frequency band passed by the filter F2. The combined output of the filter F2 and the oscillator OSC2 is applied to an amplifier A2 before being applied to a second balanced modulator MDz which is associated by means of a coupling stage 01 with a suitable oscillator OSCs.

Among the modulation products of the modulator MDZ those are chosen by means of a filter F3 which correspond to a band of frequencies comprised, for example, within the limits of 265 to 269 kilocycles.

This band of frequencies is amplified by means of a suitable amplifier A3 whose currents are applied to a harmonic generator employed at the same time as modulator HGM. On the input circuit of the harmonic generator modulator HGM are applied the frequency produced by the master oscillator MOSC by means of a coupling circuit C2 and the frequency band representing the signals to be transmitted and the pilot signal or signals provided by the amplifier A3. The apparatus HGM comprises an arrangement of vacuum tubes somewhat similar to an ordinary harmonic generator, but this apparatus is moreover adapted to operate at the same time as modulator. The object of the apparatus HGM is to modulate a suitable harmonic of the oscillations produced by' the master oscillator MOSC with the output of the amplifier A3, the arrangement being such that the corresponding side bands with or without carrier wave, that is without the considered harmonic of the frequency of the master oscillator, are found in the output circuit of said apparatus HGM. It is as a rule preferable to suppress the carrier wave in the output circuit of the apparatus HGM by employing for the latter a balanced vacuum tube arrangement.

For example, when a single vacuum tube is employed as harmonic generator, the amplitude of the desired harmonic produced varies according to the grid bias of the tube, and it is possible to find a region in which the output of the tube varies substantially linearly with the grid bias.

If, now the biasing potential of the grid be adjusted so as to correspond approximately to the middle of the linear portion of this characteristic, and if the output of the amplifier A3 be then applied to the grid, it will be seen that the amplitude of the desired harmonic will vary linearly with the instantaneous voltage supplied by the amplifier A3; in other words, the output of the amplifier A3 is linearly modulated by said desired harmonic. It will be noted that this harmonic is a harmonic of the oscillations produced by the master oscillator MOSC associated with the apparatus HGM. Consequently the currents supplied by the apparatus HGM contain the frequencies supplied by A3 modulating the desired harmonic of the master oscillator MOSC, that is to say the carrier wave. It is possible by employing an arrangement of balanced vacuum tubes for the apparatus I-IGM, to suppress the harmonic itself (that is to say the carrier wave) in its output circuit, and to keep only one or two of the modulation side-bands corresponding to this harmonic.

A filter F4 is provided at the output of the apparatus I-IGM to choose the desired side-band; this side-band contains the various frequencies supplied by the apparatus A3 and particularly the pilot wave produced by the oscillator OSC2.

In order to obtain the necessary power, a series of power amplifiers such as the amplifiers PAI, PA2, PAIl, are provided and associated with the transmitting antenna AT. It will thus be seen that the system which has just been described permits of the transmission in the case of short waves of a single side-band accompanied by a pilot Wave allowing, as will be seen in the following, the control at the receiving station of the operative conditions of the oscillator employed at this station to reproduce the carrier wave of the transmitting station. The harmonic generator modulator apparatus is based on the fact that the operation of a tube as modulator and as generator is made on the same portion of the characteristic and consequently the same tube may be employed for the two functions.

At the receiving station the received signals are applied to a receiving antenna AR and thence to a high frequency amplifier HFA, whose current is applied to a first detector DECl associated with a beat frequency oscillator controlled by quartz OSC The output of the detector is applied to an intermediate amplifier AMII whose output is applied to a band filter BF. An attenuator ATT is provided to control easily the power level in the output circuit of the filter BF. This attenuator being followed by a second intermediate amplifier AMI2 whose current is applied to a second detector DECz, an amplifier A1 if provided after this detector and a filter F1 enables the frequency band applied to the demodulator DEM associated with the oscillator 001 to be limited.

Among the demodulation products produced by the demodulator DEM, a filter F2 enables the desired band to be transmitted; this filter is followed by a low frequency amplifier ABF to which is connected an ordinary telephone receiver TR.

The system shown in Fig. 2 also permits the use of the pilot wave transmitted to the transmitting station so as to control the operative balancing of the tubes.

conditions .of the beating oscillator OSCb associated with the second detector DE-Jz. Moreover, automatic means permitting of the gain control of the system are provided and associated with the'first and second detectors or demodulators.

The pilot Wave is selected in the output circuit of the detector DEC: by means of a filter F3. The frequency of this pilot signal is displaced. in the frequency spectrum so as to occupy, for example, the position corresponding to 4 kilocycles per second, by means of an oscillator OSC associated with the detector DECi. The output of this detector DEC4 is amplified by the amplifier A2 Whose output is filtered by means of a filter F4 of very sharp cut-oiffrequency. The output of the filter F4 is applied in parallel to two apparatuses F5 and Fe which are such that their attenuation curve in function of the frequency isout in the present case at 4 kilocycles, the curves having opposite slopes on each side of the point of intersection, so that if the frequency provided by F4 increases by a small amount the voltage applied the balanced detector BDEC1 is increased While that of the detector BDECz is diminished. A slight reduction in the frequency would have an opposite effect. The voltage of F5 and Fe is applied to the grids of thetubes BDECl and BDE'Cz, so that corresponding rectified currents are produced in the output circuits of these tubes andthe difference of potential betweentwo points of this circuit will be positive ornegative according to the direction of variation of the frequency beyond or on this side of 4 kilccycles. This difference of potential is applied through a circuit CR damping the rapid variations and Whichmay also have a certain time constant; thence the corresponding current applied to an apparatusAPP associated with the oscillator OSCb and adapted to control in a compensatory-mannerthe frequency of the oscillator OSCb at the time of a change in the current supplied by the apparatus CR. It will be noted that in the present case it is not desired to change the amplitude of the pilot signal to produce a change in the difference of control potential produced by the appartus BDEC1 and BDECZ. Moreover, if it were possible to employ detectors and filters F5 and F6 perefectly balanced, a change of amplitude of the pilot signal would not produce a change in the voltage of the point S2 when the pilot signal is exactly at the frequency of 4 kilocycles, the voltage of the point S2 would only be modified duringthe fluctuations of the frequency of the pilot signal on both sides of the mean value of 4 ';ilocycles. In practice it is not possible to obtain so perfect a Consequently, it is necessary to avoid the fading effects of the pilot wave by adding an automatic high speed local gain control apparatus of the pilot signal.

. fFOI this purpose a continuous difference of potential is employed which is obtained at the point S1, which is the middle point, for example, of a resistance connected between the two plates of the detectors BDEC]. and BDECz. Thispotential is applied to the detector DEC4 so as to modify in a compensatory-manner the gain of this detector and thereby the amplitude of the pilot wave applied to the apparatus F5 and Fe is maintained constant.

In addition to the local control of the amplitude of the pilot wave or signal, the system is provided with an automatic arrangement for the gain control of the signals transmitted. This system employs the pilot wave. Consequently the detector .DECs is connected in parallel with the detector DECi; the same oscillator OSC being employed for the two detectors DECs andDECi. The pilot frequency which, in the present example, is 4 kilocycles, is filtered out by filter F7 and amplified in AMP and rectified in R. The direct voltage obtained is employed to control the gain of the detector DECI very slowly by means of special delay circuits CR1, CR2, CR3, which cancel the rapid variations in the amplitude of the rectified pilot signal; these variations are due to the fading phenomenon. Such control maintains the level of the signal substantially constant and prevents rapid fading phenomena from affecting the quality'of the received signals. The apparatus CR1, CR2, may be formed of networks comprising capacities and condensers arranged so as to produce a current curve in function of time of suitable form and in particular such that the rate of change of gain of the receiver may be sufiiciently low at any moment.

Referring now to Fig. 3 there is shown a single side-band radio receiving system in which the carrierfrequency is partly suppressed at the transmitting station and used as a pilot signal; the transmitted side-band is supposed to extend between F and F+3.000 cycles;

The system shown in Fig. 3 is suitable for receiving facsimile, television or other similar signals on a single side-band; it is necessary therefore to synchronize the oscillator resupplying the carrier at the receiver, in the correct frequency, as well as in the correct phase- If there is no selective fading, as in the case of transmission on a Wave length of 6 meters, the following equipment may be used:

The receiving antenna-AR. is connected through a high frequency amplifier I-IFA to a frequency changing detector DEC1. At the output of this detector, the carrier frequency is at 20 kc;, and the side band extends from 20 to 23 kc. The speech then passes through amplifier AI and filter BF (pass-range 20 to 23 kc.) to demodulator DEM, at which point the carrier frequency resupplied through lead Z from oscillator OSCb- (frequency 20 kc.). The speech now audio frequency is amplified to the required level by amplifier AF, to which is connected an ordinary telephone receiver TR. The beating oscillator OSC1 is of the freely oscillating type and-adjusted to F+20 kc. At the output of amplifier AI leads L are branched in paraiiel to the input of filter 331% which is tuned sharply at 20 kc;, therefore passes the pilot'wave only, which pilot is constituted as explained above by the partly suppressed carrier of the transmitting station: The output of filter 31% is connected to the input of detector DEC: at which also an E. M. F. from oscillator OSCC is applied; Oscillator OSCc is first adjusted approximately to a frequencyof 24 kc. thus giving a beat note with the pilot at 4 kc. in the output of DECz. The output note of 4 kc. is now amplified by amplifier A and is then applied to the balanced demodulator BDMl, BDMz. The grid circuits of BDMI are tuned respectively to 38 and 42 and arranged to give equal plate outputs only when the applied E. M. F. is'exactly at 4 kc. If now, for some reasons, the carrier frequency in the output of filter BF is not exactly 4 kc., then the note applied to BDML will not be exactly 4 'kc., causing a voltage to appear in the demodulator output between points and b. This voltage is then applied through the time constant device CR1 to the grid of a frequency control tube FCT which varies the frequency ofoscillator OSCc (frequency 24 kc.)

by means of leads Z, in accordance with the grid bias impressed on the frequency control tube FCT; the direction of the frequency change is arranged so that the effect of any change in frequency of the pilot in the output of BF from exactly 20 kc., is made to change the frequency of oscillator OSCQ so as to restore the pilot exactly to 20 kc. By this means it is found in practice that the frequency difference between the oscillator supplying the carrier frequency at the transmitter and the carrier locally generated at the receiver can be kept less than about 15 cycles per second.

To obtain exact phase between said oscillations, coupling condensers C are provided and serve as a weak coupling between the grids of oscillator OSCb (frequency 20 kc.) and the output current from filter BF The output from filter BF is at a frequency sufficiently near to that of oscillator OSCb to synchronize the latter automatically, by the principle of autosynchronization of coupled oscillators. This type of synchronization gives further a definite phase relation between the output of oscillator OSCb and the incoming pilot wave, which phase difference may be made equal to nought as required by suitable adjustment of the constants of oscillator OSCb.

The arrangement just described in connection with Fig. 8, shows how a partly suppressed carrier wave may be used to control exactly in frequency and in phase an oscillator at a remote receiving station; an exact phase relation is maintained in this system only if the pilot wave does not fade out during even a few cycles of the controlled oscillator.

If a constant phase relation between the synchronized oscillators is to be maintained, even under bad selective fading conditions, two pilot frequencies at least should be used, so that if one fades out at a certain moment, the controlled oscillator will be kept in phase by the second pilot, since selective fading will usually not aifect equally the two pilots at the same moment.

In order to simplify the description, it will be assumed that the carrier frequency F is partly suppressed and is used as a first pilot wave; the second pilot will be supposed at a frequency of F+3xl kc. This separation between the two pilots is suflicient in general to prevent both pilots from fading out below the noise level simultaneously. The circuit as described above is used for pilot I, but in this case an automatic gain control system must be added. By this means the output of amplifier Ap at 4 kc., rectified by the detector DECs is made to control the grid bias of detector DEC2, thus, the output E. M. F. of DECz is kept substantially constant in spite of variations in the input level.

The equipment of the transmitting station may be as shown in Fig. 1 of the accompanying drawings, but in this case the carrier wave will not be entirely balanced out, so that a portion thereof can be transmitted to the receiving station and used as a pilot wave. The second pilot will be supplied by an oscillator (oscillator OSCz in Fig. l) as described in connection with Fig. 1. At the receiving station, the equipment will be as shown in Fig. 3 and will comprise the further equipment to be described below: First of all a gain control equipment comprising a detector DECa will be inserted between the demodulator DEC2 and the balanced demodulator BDMl and BDMz to keep constant the amplitude of the first pilot wave. The filter. FPz tuned sharply ,to 23.4 kc;

will be inserted in the output of amplifier AI.

This filter will select the second pilot which has a frequency equal to F+3.4 kc.

A frequency changer FG will be provided in the output of filter FPz, in which a frequency of 3.4 kc. will be produced by interaction of a frequency generated by the oscillator tube CT which is subtracted from pilot 2. A filter BFPI, placed in the output of frequency changer FG is tuned sharply to 20 kc. and provides in its output a frequency equal to that of the first pilot 1. It will be seen therefore that the second pilot, after traversing the path described above, will have its frequency brought to a value equal to that of pilot 2. The second pilot changes in frequency flow in the output of BFr-u and is applied to amplifier Al and thus to point P and also through cou: pling condenser C to control oscillator OSCb.

The oscillator CT must now be synchronized in frequency and in phase with the corresponding oscillator at the transmitting station. For this purpose, a beat note is first obtained between the two pilots and this is done in detector IBD through coupling condenser C1 for pilot 1 and C2 for pilot 2. In the output of BD there is obtained a frequency equal to 3.4 kc. which is ap-' plied through an amplifier ABF associated with a filter tuned to 3.4 kc. for selecting the beat note. At this stage there is added a separate gain control equipment comprising lead L and rectifier R. The D. C. output of rectifier R controls the grid bias of tube BD. In the output of ABF, a constant current is obtained; except when fading is so strong that the pilot considered is reduced below noise, level. The corresponding output is applied in push-pull to the grids of a differential detector DFC. These two grids are also fed in the same phase with the output of oscillator CT by transformer T2. The D. C. voltage from the difierential detector DFC is applied through a time constant device or delay network TCD, to the frequency controlling tube FCT, thus controlling its grid bias, and in turn controlling the frequency of oscillator CT. So, a frequency or phase change in the beat between the two pilot waves will cause the oscillator CF to change its frequency and/or phase so as to restore the original relation between the beating frequencies. It will be clear, from What has been described above that, if one of the pilots fades out, the oscillator OSCb supplying at the receiving station the partly suppressed carrier will have a constant frequency and phase relation to that of the carrier frequency oscillator of the transmitting station.

The time constant of detector DECs shown in Fig. 3, can be made variable according to the amplitude of pilot 1'. Since delay networks usually comprise inductances, capacities, and resistances and'since the amount of delay provided depends upon the values chosen for these elements, it is clear that a delay variable with the amplitude of the pilot wave may be provided by means of a delay network, one element (inductances, capacity, resistance) of which at least is adapted to vary in accordance with the pilot wave. For instance the inductance of the delay network may be varied by means of a winding traversed by the pilot current and adapted to saturate the core of said inductance; this core may be a permalloy core. The inductance may also be varied by controlling the position in the inductance core in accordance with the amplitude of the pilot wave. Similar means could be used for controlling the capacity element of the delay network. Finally the'delay may be varied by controlling the resistance element of the network; this may be done for instanceby associating with said 'reaccordance with the amplitude of the second pilot.

What is claimed'is:

1. A signaling system, subject to fading phenomena, comprising a local oscillator at a receiving stationadapted to be controlled from a transmittingstation by a pilot wave, and means at said receiving station adapted to continue the control on said oscillator during fading Periods when the pilot wave is not received at the receiving station. V I I 2. A signaling system comprising a transmitting station, a receiving station, a local oscillator at' 'said'receiving station, means at said transmitting station'for producing a pilot wave adapted to control'said local oscillator, and delay devices associated with said local oscillator and adapted to be controlled in accordance with the amplitude/of said pilot wave.

'3. A signaling system comprising means at'a transmitting station for producing a pilot wave, an oscillator adapted to 'be controlled by said pilot Wave andgain control equipment at a receiving station, and delay means associated with said equipment adapted'to be controlled in accordance with the change in amplitude of said pilot wave due to fading phenomena existing between said stations.

4 A signaling system comprising a transmitting station, means for generating and transmitting therefrom a signal modulated wave and two pilot waves, a receiving station, a local oscillator thereat adapted to produce a wave having the same frequency as one of said pilot waves, and means at said station wherebya wave ofthe frequency ofthe' other of said pilot waves is obtained from said oscillator. Y

5. A signaling system comprising a transmitting station, a signal source, a high frequency oscillator, means for modulating the high frequencyby the signals, means for partially suppressing the unmodulated high frequency wave, means for transmitting the modulated wave and the partially suppressed unmodulated Wave, and

a receiving station having a local oscillator, adapted to oscillate at the same frequency as said high frequencywave and means whereby the unmodulated high frequency wave received is adapted to control said local oscillator so that it will produce the same frequency as saidhigh frequency wave, and keep in a constant phase relation with the, oscillator at said transmitting station.

"6.'A "signaling system, subject to fading phenomena, comprising an oscillator at a transmittingstation and an oscillator at a receiving station, the former oscillator being adapted to control the latter, and means whereby during fading periods the diiference of frequencies between the two oscillators does not exceed a predetermined limit beyond which the distortion of the transmitted signals would be intolerable.

"7. A signaling system comprising a receiving station adapted to receive signal waves and a pilot. wave, said station having a path for said pilot wave including a quick acting gain control equipment and a path for said signal waves in-.

cluding a comparatively slowacting gain control equipment.

8. Ai i naling system subject" to'fading phenomena comprising a local oscillator at a receiving station adaptedto be controlled from a transmitting station by aplurality of pilot waves, more than one being used to guard against the possibility of non-reception of a pilot wave which might be the case during fading periods, and separate paths for each of said pilot waves each including a gain control equipment. 1

9. A signaling system comprising a transmitting station having means for transmitting asignalwave and a plurality of pilot waves therefrom, more than one pilot wave being provided to guard against the possibility of the non-reception of a pilot wave in the event of fading, and a receiving station having means for detecting said waves, the means. for detecting said sig-. nal wave being controlled. by at least one of said pilot waves, the pilot waveexerting this control at any given time depending upon the electrical conditions existing, between said stations.

10. Asignaling system using modulated waves wherein at least one product of modulation, such as a side band, is suppressed at the transmitting station and the carrier wave is also suppressed or considerably attenuated before transmission in which a pilot frequency depending upon the carrier frequency istransmitted to the receiving station and used for automatically regulating the frequency of a locally generated carrier fre-- quency characterized in this that means located at the transmitting station for transmitting said pilot wave as a train of impulses.

12. A system in accordance with claim 10 characterized in this that means are provided at the transmitting station for emitting said pilot" wave in the form of a sustained oscillation at comparatively high frequency modulated by a comparatively low frequency oscillation. I 13. A signaling system of the singleband type comprising a transmitting station, a receiving station, means for transmitting a signal carrier wavefrom said transmitting station to said'rea' ceiving station, 1 means for transmitting a pilot Wave, havinga frequency depending upon the frequency; of said signal carrier wave, from the transmitting to the receiving station, a local oscillator at the receivingstation forresupplying the carrier wave, means for causing said pilot wave to exert a controlling effect on said local oscillator and means comprising quick acting gain control equipment adaptedto vary said of v feet in accordance with the amount of attenua-- tion imposed on the signals by the fadingphenomena to which the system'is subjected. I I 14. A signaling system according to claim 13,

in which meansare providedat the receiving station and adapted to select the pilot wave and the-receivedside band characterized in this, that a comparatively slow acting gain control equip ment is provided in the path of the received side;

band.

15, A system according to claim 13 wherein means are provided at the transmitter for send ing a plurality of pilot-waves to the receiving-station for controlling the frequencyof a local oscil-,

latorused for-resupplying the carrier wave, morethanone-pilot wave being provided so as to guard against the possibility of the non-reception of a pilot wave which, under fading conditions, might result if only one pilot wave were used, characterized in this that further means are provided for selecting each one of said pilot waves and for applying them individually to different paths in which individual gain control equipments are provided.

16. A system according to claim 13 wherein means are provided at the transmitter for sending a plurality of pilot waves to the receiving station for controlling the frequency of a local oscillator used for resupplying the carrier wave, more than one pilot wave being provided so as to guard against the possibility of non-reception of a pilot wave which under fading conditions might result if only one pilot wave were used, means at the receiving station for selecting each one of said pilot waves and for applying them individually to difierent paths in which individual gain control equipments are provided, and means whereby each of said control equipments are adapted to have different time delays.

1'7. A signaling system subject to fading phenomena comprising a transmitting station, a receiving station, an oscillator at said receiving station, means at said transmitting station for producing a plurality of pilot waves, more than one pilot wave being provided to ensure that, if fading exists between said stations, at least one pilot wave will not become obliterated and will be received at the receiving station, means at said receiving station adapted to change the frequency of one of said pilot waves, and means whereby said changed frequency is adapted to control said oscillator.

18. A system in accordance with claim 1'7 having an oscillator at the transmitting station, an oscillator at the receiving station intended to operate in synchronism with the oscillator at the transmitting station, and means for producing a beat note between said pilot waves for effecting such synchronism.

19. A system in accordance with claim 17 characterized in this that means are provided at the transmitting station whereby the frequencies of said pilot waves are regularly spaced in the frequency spectrum.

20. A signaling system comprising a transmitting station, a high frequency oscillator thereat, a signal source, means for modulating the high frequency produced by said oscillator by the signals and for transmitting the modulated wave, means for also transmitting a pilot wave and a receiving station having means controlled by said pilot wave for efiecting gain control of the received signals and having a local oscillator with which is associated means under the control of the pilot wave, adapted to compensate for fading effects between said stations.

21. A system in accordance with claim 20, wherein. gain control equipment is provided in the path of the pilot wave and means under the control of the amplitude of the pilot wave for delaying the operation of said gain control equipment.

22. A system in accordance with claim 20, characterized by means for rendering gain control equipment in the path of the signals variable in accordance with the average amplitude of received signals.

23. A system in accordance with claim 20 wherein means is provided for transmitting a plurality of pilot waves to said receiving station, more than one pilot wave being supplied in order to ensure that, when fading exists between said stations, at least one of the pilot waves will reach the receiving station at a suficient amplitude to be detected, means at the receiving station for converting the received pilot waves to a predetermined frequency, and means whereby said frequency is adapted to control said oscillator.

ALEC HARLEY REEVES. 

